1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to technology for fixing and holding a recording medium and technology for conveying the recording medium in an image recording device for forming an image on the recording medium.
2. Description of the Related Art
As a general image forming apparatus, it is suitable to use an inkjet recording apparatus, which forms a desired image on a recording medium by ejecting and depositing a plurality of colors of inks onto the recording medium from a plurality of nozzles provided in an inkjet head. In the inkjet recording apparatus, the recording medium is held and conveyed by, for example, a drum conveyance method or a belt conveyance method. In the drum conveyance method, the recording medium is held on the outer circumferential surface of a conveyance member having a drum shape, and the recording medium is conveyed with rotation of the drum. In the belt conveyance method, the recording medium is held on the surface of an endless belt that is wrapped about a plurality of rollers, and the recording medium is conveyed with rotation of the rollers. As the method of holding the recording medium on the conveyance member, one of various methods is used appropriately in accordance with the composition of the apparatus: such as an air suction method, in which the recording medium is fixed and held by a suction pressure (negative pressure) applied through suction holes arranged in the surface of the conveyance member on which the recording medium is held from the inner side of the conveyance member; an electrostatic attraction method, which uses static electricity; a method which uses a mechanical holding member; and the like.
In the inkjet recording apparatus, in order to form images of high definition it is necessary to bring the inkjet head and the recording medium as close together as possible during image formation. On the other hand, if the inkjet head and the recording medium make contact with each other, then the image formed on the recording medium may be degraded and the inkjet head may be damaged. Hence, an extremely small working distance of several millimeters or less is allowed between the inkjet head and the recording medium, with the object of avoiding contact between the inkjet head and the recording medium.
Japanese Patent Application Publication No. 10-175337 discloses an inkjet printer in which a print medium which is electrostatically charged by a charging roller is then held on the outer circumferential surface of a drum by an electrostatic force of attraction. The drum is rotatable about a prescribed axis, and printing is carried out by depositing ink onto the print medium while the drum rotates. This inkjet printer adopts a composition in which the leading end portion of the print medium is held by applying a mechanical gripping force by means of a gripping hook at a position to the inside of the outer circumferential surface of the drum. The gripping hook is arranged in the center of a recess section formed on the outer circumferential surface of the drum so as to prevent the gripping hook from projecting beyond the outer circumferential surface of the drum and interfering with the nozzle unit (inkjet head).
However, if a recording medium is held in a bent state whereby the leading end portion of the recording medium is held to the inside of the outer circumferential surface of the drum, as in the inkjet printer described in Japanese Patent Application Publication No. 10-175337, the leading end portion of the recording medium (the vicinity of the bent portion of the recording medium) may lift up above the drum.
FIG. 22 shows a schematic view of the lifting up of a recording medium in the drum conveyance method described above. As shown in FIG. 22, when a leading end portion 614A of a recording medium 614 held on an outer circumferential surface 606 of a conveyance drum 600 is held by means of a gripper 616 to the inside of the outer circumferential surface 606 of the conveyance drum 600, then a bent portion 614B of the recording medium 614 floats up from the outer circumferential surface 606 of the conveyance drum 600.
If lifting up of the recording medium occurs in this way, then the portion where the image cannot be satisfactorily formed at the leading end side of the recording medium becomes larger, and it is necessary to dispose the inkjet head at a distance of separation from the recording medium which is greater than the amount of lifting up of the recording medium, and this is disadvantageous in terms of high-quality image formation.